Conventional golf balls can be divided into two general classes: solid and wound. Solid golf balls include one-piece, two-piece (i.e., single layer core and single layer cover), and multi-layer (i.e., solid core of one or more layers and/or a cover of one or more layers) golf balls. Wound golf balls typically include a solid, hollow, or fluid-filled center, surrounded by a tensioned elastomeric material, and a cover.
Examples of golf ball materials range from rubber materials, such as balata, styrene butadiene, polybutadiene, or polyisoprene, to thermoplastic or thermoset resins such as ionomers, polyolefins, polyamides, polyesters, polyurethanes, polyureas and/or polyurethane/polyurea hybrids, and blends thereof. Typically, outer layers are formed about the spherical outer surface of an innermost golf ball layer via compression molding, casting, or injection molding.
From the perspective of a golf ball manufacturer, it is desirable to have materials exhibiting a wide range of properties, such as resilience, durability, spin, and “feel,” because this enables the manufacturer to make and sell golf balls suited to differing levels of ability and/or preferences. In this regard, playing characteristics of golf balls, such as spin, feel, CoR and compression can be tailored by varying the properties of the golf ball materials and/or adding additional golf ball layers such as at least one intermediate layer disposed between the cover and the core. Intermediate layers can be of solid construction, and have also been formed of a tensioned elastomeric winding. The difference in play characteristics resulting from these different types of constructions can be quite significant.
Unfortunately, golf ball polymer compositions can begin to deteriorate as early as during golf ball manufacture due to the processing conditions under which golf balls are typically made. Deterioration then continues as the material weathers when exposed to environmental conditions such as sunlight (UV light/rays) on the course. UV light/rays can initiate deteriorating photochemical processes in golf ball polymers containing UV absorbent groups or impurities. Weathering impacts not only the golf ball's appearance but its durability, including creating poor adhesion between adjacent layers and reducing impact strength of outermost surfaces.
Golf ball manufacturers tend to incorporate coloring agents such as titanium dioxide (TiO2) in ionomers in order to impart a suitable color to the material which would otherwise generally be colorless. UV light can deteriorate inner or outer surface properties in colored ionomers. In rubber materials, destructive peroxy radicals are known to form during the rubber degradation process, and aromatic isocyanate-based polyurethane and polyurea polymers are particularly vulnerable to weathering from exposure to UV light rays since aromatic structures are inherently unstable and may be found in the reaction product.
Golf ball manufacturers typically address these problems by incorporating stabilizers in golf ball polymer compositions. Conventional antidegradants including UV absorbers, radical scavengers, peroxide decomposers, and quenchers can afford some protection to polymers against the harmful effects of degradation. Each of these classes of antidegradent plays a unique role in protecting a golf ball polymer from a specific cause of deterioration.
For example, UV absorbers are generally helpful to absorb or filter damaging light before a chromosphore (the part of a molecule responsible for its color) can be formed. UV absorbers can absorb harmful UV light and transform it into harmless heat. Examples include 2-(2-hydroxyphenyl)-benzotriazoles, 2-hydroxy-benzophenones, hydroxyphenyl-s-triazines, and oxalanilides, each of which are characterized by a specific absorption and transmission spectrum. A suitable UV absorber should absorb UV light better and faster than the polymer it is added to protect against, and dissipate absorbed energy before undesirable side reactions occur.
In turn, peroxide decomposers decompose peroxides into non-radical and stable products, and quenchers accept energy from excited polymer molecules through an energy transfer mechanism and deactivate chromosphores before the excited states can undergo a reaction resulting in degradation. On the other hand, free radical scavengers can trap radicals before undesirable reactions (polymer degradation) takes place. Suitable free radical scavengers should be capable of trapping radicals and interrupting the chain reaction that can occur in a polymer when an excited chromophore decomposes to form radicals. Free radicals typically (i) react with the polymer and/or atmospheric oxygen, or (ii) remove a hydrogen atom from the polymer thereby initiating a free radical reaction. Examples of conventional free radical scavengers include sterically hindered amines (HALS) and antioxidants. HALS are typically derivatives of 2,2,6,6-tetraamethylpiperidine and react with a free radical to give the stable nitroxyl radical.
Meanwhile, antioxidants can potentially prolong the service life of a broad range of polymers. Common primary antioxidants include amines and phenolic antioxidants, which are chain terminating. Phenolic antioxidants are often used to inhibit thermo-oxidation at higher processing temperatures (e.g., ≥150° C.) and catalyze formation of a stable phenoxy radical to terminate free radical chain reactions initiated in a polymer. Secondary antioxidants, e.g., phosphites, can decompose peroxide.
Given these different roles, “stabilizer packages” comprised of antidegradants from several different classes are often included in golf ball polymer compositions. One drawback with conventional stabilizers, however, is their tendency to shift or migrate within a polymeric material over time, thereby limiting the degree and shortening the lifespan of protection provided by the stabilizer against weathering—which negatively impacts golf ball durability. This shift can be inward toward/into an inner adjacent golf ball layer or outward toward the layer's surface and/or an adjacent outer layer.
Thus, there is a need for golf balls possessing longer term protection against weathering that may be produced cost effectively within existing golf ball manufacturing processes. Golf balls of the present invention and method of making same address and solve this need.